A liquid crystal display is explained as a representative of flat-panel displays that have come to be widely adopted in recent years. A liquid crystal display is used widely in applications ranging from a portable phone to a large-size television exceeding 100 inches. Such a liquid crystal display is categorized into a simple matrix liquid crystal display and an active matrix liquid crystal display by the method for driving pixels. In the two kinds of liquid crystal displays, the active matrix liquid crystal display incorporating a thin film transistor (abbreviated as TFT in this description) as a switching element is the main stream in the current liquid crystal displays since it has a high image quality and a good conformity with high speed moving images.
A liquid crystal display device of an active matrix liquid crystal display comprises: a glass-made TFT module substrate; a glass-made opposing substrate installed opposite the TFT module substrate; and a liquid crystal layer functioning as an optical modulator installed between the TFT module substrate and the opposing substrate. Here in the description, explanations are hereunder based on the premise that the TFT module substrate (the glass substrate) is located on the lower side and the opposing substrate is located on the upper side in an liquid crystal display device.
FIG. 1 shows a cross-sectional view of a TFT element 1 for a flat-panel display composing such a liquid crystal display device. A wiring film 2, a transparent conductive film (a transparent pixel electrode film) 5, a TFT 6, and others are installed on the upper surface of a glass substrate (a TFT module substrate) 3. The transparent conductive film 5 is an indium tin oxide (ITO) thin film, an indium zinc oxide (IZO) thin film, or the like. Meanwhile, the glass substrate (the TFT module substrate) 3 is driven by a driver circuit and a control circuit connected through a TAB tape.
Although the following units are not shown in the figure, on the bottom surface of the opposing substrate facing the TFT module substrate, a common electrode extending all over the surface, a color filter placed at a position facing the transparent conductive film, and a light-shielding film placed at a position facing the TFT and the like are installed. In addition, an orientation film to orient liquid crystal molecules contained in the liquid crystal layer into a predetermined direction is installed. Further, polarizing plates are installed on the bottom surface side of the TFT module substrate and the upper surface side of the opposing substrate, respectively.
A backlight is installed at the lower part of the liquid crystal display device constructed as stated above and the light passes through from the TFT module substrate side to the opposing substrate side. At each of unit pixels in the liquid crystal layer of the liquid crystal display, the electric field between the opposing substrate and the transparent conductive film (the transparent pixel electrode film) is controlled by the TFT, the orientation of liquid crystal molecules changes by the electric field in the liquid crystal layer, and the light passing through the liquid crystal layer is modulated (shielded or transmitted). By so doing, the quantity of the light transmitted through the opposing substrate is controlled and an image is displayed.
As shown in FIG. 1, a conventional TFT element 1 for a flat-panel display has a wiring film (scanning lines) 2 comprising an aluminum alloy film on the upper surface of a TFT module substrate 3 and a part of the wiring film 2 functions as a gate electrode 7 to control ON/OFF of a TFT 6. Further, a signal line 9 comprising an aluminum alloy film is formed in the direction perpendicular to the wiring film 2 on the upper surface of a gate insulating film 8 installed so as to cover the wiring film 2. A part of the signal line 9 functions as a source electrode 10 of the TFT 6. A transparent conductive film (a transparent pixel electrode film) 5, for example, comprising ITO is formed in the pixel region on the gate insulating film 8. A drain electrode 11 of the TFT 6 comprising an aluminum alloy film formed on the upper surface of the gate insulating film 8 electrically contacts the transparent conductive film 5.
Firstly, when a gate voltage is applied to the gate electrode 7 through the wiring film 2, the TFT 6 is turned on, a drive voltage applied to the signal line 9 beforehand is applied to the transparent conductive film 5 from the source electrode 10 through the drain electrode 11, a sufficient amount of potential difference is generated between the transparent conductive film 5 and the common electrode in the opposing substrate when a drive voltage of a predetermined level is applied to the transparent conductive film 5, and the liquid crystal molecules comprised in the liquid crystal layer are oriented and optical modulation is caused.
As explained above, a conventional wiring part such as a wiring film including a gate electrode has been made of a film of a high-melting-point metal, such as Mo or Cr, and an aluminum alloy film. As a liquid crystal display is getting larger in size and higher in image quality however, the electrical resistance of the wiring film increases, problems such as signal delay, electric power loss, and others appear, and therefore copper (Cu) of a lower electrical resistivity than aluminum (Al) draws attention.
Incidentally, the electrical resistivity of an Al-2 atomic % Nd thin film used as a conventional material is 4.1×10−6 Ω·cm after heat treatment is applied at 350° C. for 30 minutes in vacuum and the electrical resistivity of a Cu thin film is 2.0×10−6 Ω·cm after heat treatment is applied at 350° C. for 30 minutes in vacuum. Problems such as peel off do not appear even when an Al wiring film is formed directly on a glass substrate. When a Cu wiring film is formed on a glass substrate however, the problem has been that the Cu wiring film peels off from the glass substrate since the adhesiveness between glass and Cu is poor.
As patents to solve the problems, various technologies described in Patent Documents 1 to 9 are proposed.
In the technology described in Patent Documents 1 and 2, a layer of a high-melting-point metal such as molybdenum (Mo) or chromium (Cr) is interposed between a Cu wiring film and a glass substrate. By forming such a primary layer as excellent in adhesiveness, the adhesiveness of the wiring film itself is improved and the buildup or the breakage of the Cu wiring film is prevented when patterns are formed. However arising new problems have been that not only a process to form a high-melting-point metal layer as the primary layer is added but also, since two different metals of Cu and the high-melting-point metal are layered, corrosion occurs at the interface between Cu and the high-melting-point metal when a chemical solution is applied for etching and a desirable cross-sectional shape of the wiring film (it is considered that the desirable taper angle is 45 to 60 degrees) is not formed because of the difference of the etching rates in the layers. Another problem has been that a process related to forming patterns of the Cu wiring film by using an etching liquid suitable for each of the layers has to be added in order to form a desirable cross-sectional shape of the wiring film and thereby the production cost of a liquid crystal panel increases.
In the technology described in Patent Document 3, it is said that adhesiveness improves by coating a glass substrate with a chromium (Cr) film as a primary layer, forming a Cu layer comprising Cr thereon, and separating into Cu—Cr two layers by applying heat treatment. Even by this technology however, it is very difficult to adjust the cross-sectional shape of a wiring film. Moreover, the electrical resistivity of Cr is 12.9×10−6 Ω·cm, which is higher than the electrical resistivities of Cu and Al-2 atomic % Nd, and thus problems of signal delay and electric power loss caused by the increase of the wiring resistance occur.
In the technology described in Patent Document 4, it is said that adhesiveness can be improved by forming a resin layer and a layer of a metal such as Cr or the like between a glass substrate and a Cu wire. The problem of the technology however is that the resin layer deteriorates by the thermal history in the production process of a liquid crystal display and the adhesiveness rather deteriorates.
In the technology described in Patent Document 5, it is said that the adhesiveness of a glass substrate is improved by using an alloy produced by adding gold (Au) or cobalt (Co) to Cu. However, although a Cu—Au alloy thin film and a Cu—Co alloy thin film are more excellent than a Cu thin film in adhesiveness, the adhesiveness is still insufficient for a liquid crystal display (actual peel test data of a Cu—Au alloy thin film and others are described in Examples).
In the technology described in Patent Document 6, silicon (Si) that is the main component of a glass substrate is likely to react with Cu, thus the electrical resistance of a Cu wire possibly increases, and therefore a Cu film comprising nitrogen (N) (CuxN) is firstly deposited on the upper surface of a glass-made transparent substrate by sputtering, and then a Cu layer is formed. Further, in the technology described in Patent Document 7, it is proposed to use a Cu wiring film comprising nitrogen (CuxN) in order to prevent the Cu wiring film from oxidizing and improve the adhesiveness to the substrate. Since the CuxN itself is not a stable compound however, the problem of both the technologies described in Parent Documents 6 and 7 has been that N atoms diffuse in the Cu wiring film through the thermal history in the production process of a liquid crystal display and the electric resistance of the wire increases.
In the technology described in Patent Document 8, a Cu wiring film is formed by firstly coating the upper surface of a glass substrate with a silicon nitride film, successively vapor-depositing a primary metal layer comprising tantalum (Ta) or the like, further forming a metal seed layer comprising Cu or the like thereon, and finally applying nonelectrolytic plating. The problems of the technology however are that the number of processes to form a wiring film is many, the processes are so complicated as to be optimized, a wet process is included, and thus contamination by impurities and necessity of waste treatment arise.
In the technology described in Patent Document 9, it is proposed to form a wiring pattern with craft polymer having electrical conductivity and successively form a Cu wiring film by nonelectrolytic plating. The problem of the technology however is that the craft polymer (a resin layer) deteriorates through the thermal history in the production process of a liquid crystal display and the adhesiveness rather deteriorates.    Patent Document 1: JP-A No. 66423/1995    Patent Document 2: JP-A No. 8498/1996    Patent Document 3: JP-A No. 138461/1996    Patent Document 4: JP-A No. 186389/1998    Patent Document 5: JP-A No. 342563/2003    Patent Document 6: JP-A No. 212940/2004    Patent Document 7: JP-A No. 133597/1998    Patent Document 8: JP-A No. 24754/2006    Patent Document 9: JP-A No. 108622/2006